SEDIMENTOLOGY OF THE REVISED SONSELA MEMBER OF THE CHINLE FORMATION, PETRIFIED FOREST NATIONAL PARK, ARIZONA

Recent mapping of Petrified Forest National Park (PEFO) has shown that the Sonsela Member of the Chinle Formation within PEFO, like the type section, consists of a tripartite subdivision of lower and upper sandstone beds and an intervening mud-dominated interval. The lower sandstone interval consists of the informal Rainbow Forest beds. The informal Jim Camp Wash beds comprise the medial mud-dominated interval. The upper sandstone interval consists of the informal Flattops 1 bed. All three of these stratigraphic intervals have been previously described in the literature, but have only recently been associated with the Sonsela Member. Lateral continuity, composition and depositional style unify all three units within the Sonsela, and distinguish the Sonsela from other members of the Chinle within PEFO.

A sedimentologic model that incorporates both the sandstone and mudstone facies was constructed during the course of this study. The Rainbow Forest beds represent a braided river system that cut broad, shallow scours into the underlying dominantly overbank sediments of the Blue Mesa Member. Paleocurrents indicate flow directions towards the northeast, in contrast to the predominantly northwesterly flow of the Blue Mesa Member. The Rainbow Forest beds grade up into the Jim Camp Wash beds.

The Jim Camp Wash beds represent a moderate sinuosity stream system that underwent frequent avulsions. Individual channels are locally preserved as sandstone lenses up to 15 meters thick. Mudstone and heterolithic avulsion deposits form the majority of sediments in the interval. Paleoflow is also predominantly to the northeast. The Jim Camp Wash beds grade into or are locally overlain by a shallow scour surface at the base of the overlying Flattops 1 bed.

The Flattops 1 bed is interpreted as a return to lower sinuosities and predominantly channel sandstone deposition. The lower portion is low sinuosity, while the upper portion shows evidence, such as scroll-bar topography, of moderate to high sinuosity. Paleoflow is variable from easterly to northerly. This unit includes what has previously been referred to as either the Flattops 1 bed or the Sonsela Sandstone, including the sandstones that cap Blue and Agate mesas. The Flattops 1 bed grades up into the overlying Petrified Forest Member and a return to northwesterly flow directions.